1. Field of the Invention
The present invention relates to a control apparatus for controlling a hydraulically operated anti-lock brake system for a motor vehicle, so as to prevent braked wheels from excessively slipping on a road surface.
2. Discussion of the Prior Art
An anti-lock or anti-skid brake control apparatus is generally constructed to include (a) a pressure regulating valve device disposed between a hydraulic pressure source and brake cylinders of wheel brakes provided to apply brake to vehicle wheels, for controlling the brake fluid pressures in the brake cylinders, (b) slip detecting means for detecting a slip of each wheel, and (c) control means for controlling the pressure regulating valve device based on the output signal of the slip detecting means, so as to prevent excessive slipping of the wheels under braking. The degree of slip of each vehicle wheel is represented by a difference between the running speed of the vehicle and the rotating speed of the relevant wheel, or a ratio of this difference to the vehicle speed. Alternatively, the slip of the wheel is represented by a deceleration value (negative acceleration value) of the wheel.
An example of the pressure regulating valve device is disclosed in U.S. Pat. No. 4,668,024, which is a three-position valve having a pressure-hold position for maintaining the pressure in the brake cylinder, as well as a pressure-increase and a pressure-decrease position for increasing and decreasing the brake cylinder pressure, respectively. Another type of the pressure regulating valve device is disclosed in U.S. Pat. No. 4,842,343, which is a combination of a directional control valve for increasing and decreasing the brake cylinder pressure, and a shut-off valve for changing the rate of flow of the fluid into and from the brake cylinder, so that the brake cylinder pressure is controlled in a selected one of four modes, i.e., rapid increase mode, slow increase mode, rapid decrease mode and slow decrease mode.
The known pressure regulating valve devices as described above are necessarily complicated in construction and accordingly expensive. While the use of a two-position directional control valve having only the pressure-increase and pressure-decrease positions may be a solution to this problem, it is not easy to adequately control the two-position valve. Described more specifically, the brake cylinder pressure should usually be decreased at a rate higher than the rate at which the pressure is increased. In the absence of a pressure-hold or slow-decrease position, the two-position directional control valve is not capable of lowering the rate of decrease in the brake cylinder pressure for a terminal portion of the pressure-decrease period prior to resuming an increase in the brake cylinder pressure. This incapability of lowering the pressure-decrease rate results in an excessive decrease of the brake cylinder pressure, due to a control delay, if the valve is switched to the pressure-increase position when a sufficient degree of reduction in the wheel slip ratio or a symptom of such sufficient reduction in the slip ratio is actually detected. In other words, the two-position pressure regulating valve is difficult to control for obtaining an optimum amount of reduction in the brake cylinder pressure during the anti-lock brake pressure regulating operation.